In the manufacture of devices of reinforced plastic, the use of resin impregnated filaments such as, for example, resin coated glass roving is well known, especially for use in laminating, molding and filament winding techniques. Filament winding typically involves coating or impregnating glass roving, yarn or the like with liquid resin and a suitable curing agent for the resin. The roving can be dipped into the liquid mixture, or otherwise brought into contact with it. The impregnated roving can then be wound, layer upon layer, onto a mandrel having suitable configuration to provide a preform of the desired product. The preform can then be cured by application of heat (and possibly pressure) over a period of time to yield the desired product. Use of resin impregnated filaments are likewise well known in molding and laminating applications, which also involve curing by application of heat (and possibly pressure) for a suitable length of time.
Numerous classes of resins suitable for use in such laminating, molding and filament winding techniques are known to the skilled of the art. These include, for example, epoxide resins, silicone resins, polysulfides, polyurethanes, and polyester resins. As referred to herein, polyester resin means unsaturated polyester resin which can be further polymerized, many of which are well known to the skilled of the art. Tetraethylene glycol dimethacrylate and like materials, in polymerizable form, are also taken to be within the scope of the term polyester resin. Unsaturated polyester resins often include, as a diluent, an unsaturated monomer such as, for example, styrene. It is well known in the art that unsaturated polyester resin cures or continues its polymerization reaction by a free-radical mechanism which is generally self-sustaining after initiation. Initiation of the curing reaction is accomplished by contacting the unsaturated polyester resin with a suitable initiator, often referred to as a curing agent. Well known curing agents suitable for polyester resins include certain organic peroxides such as, for example, benzoyl peroxide, methylethylketone peroxide, cumene hydroperoxide and dichlorobenzoyl peroxide.
As has been noted, the free-radical polymerization mechanism of the polyester resins is self-sustaining once initiated by the curing agent. Thus, curing does not strictly require uniform mixing of the curing agent with the polyester resin. The length of time needed for cure will be greater, however, where the curing agent is not uniformly dispersed throughout the resin and where, consequently, initiation of the polymerization reaction does not occur uniformly throughout the resin. This is especially so in view of the slow diffusion of the relatively large peroxide molecules of the curing agent through the resin. In addition, since the polymerization reaction proceeds faster at higher temperatures and is exothermic, non-uniform mixing of the curing agent into the resin can result in non-uniform curing, increased internal stress and consequently less desirable physical properties in the product.
Even where the curing agent is uniformly mixed into the polyester resin, curing is typically very slow at room temperature. Heating the resin accelerates curing. Thus, curing of thermosetting polyester resins is typically carried out at a temperature of from about 50.degree.-175.degree. C. Naturally, however, application of heat to achieve increased speed of curing is a significant disadvantage. Not only does the need for heat generation equipment and energy add substantial expense to the manufacturing process, but in addition, it necessarily limits production capacity to that of the ovens or other heat generating equipment. While compositions of resins and curing agents are known which are curable at room temperature, these typically require a substantially longer curing period, which itself adds considerable expense and limits production capacity.
Additional problems are involved in the use of heating in the manufacture of structural devices having large cross-sectional areas. Heating creates a temperature gradient through the pre-form, and since curing proceeds at a rate proportional to resin temperature, curing of the resin inside the pre-form is slow. That is, as a consequence of the temperature gradient, curing proceeds at a different rate throughout the pre-form. Non-uniform cure produces internal stresses in the cured product, which consequently, will have reduced material strength. Thus, for example, the skilled of the art would expect a leaf spring for an automotive vehicle suspension system, made by such conventional system to be unable to withstand normal use. While the normal curing process of thermosetting polyester resin is exothermic, the heat generated internally during cure typically is insufficient to cause curing to proceed sufficiently rapidly to overcome these adverse effects of the temperature gradient resulting from external heating during cure of a pre-form having a large cross-section. Heat transfer is poor through the uncured resin and, therefore, while external heat can be applied to accelerate curing, a temperature gradient across the pre-form cross-section will result.
Accordingly, it is known in the art to reduce curing time and/or heating requirements by admixing an accelerator with the resin and curing agent. Accelerators are known which are suitable for use with a given resin and curing agent. In the case of polyester resins, for example, known accelerators suitable for use in admixture with a diacyl peroxide curing agent include materials such as, for example, diethylaniline. Once an accelerator is incorporated into the polyester resin system, curing will proceed faster and/or at a lower temperature. While advantageous for the reasons already mentioned, this also presents serious disadvantages. In particular, such accelerated resin systems have greatly reduced pot life. In addition, the physical properties of a batch of resin will change constantly during use due to its constantly increasing degree of cure, i.e., polymerization at the time of use. Consequently, there results both waste of resin not timely used and diminished product quality. Attempts to avoid the problems of short pot life and changing resin properties have included the suggestion in U.S. Pat. No. 3,390,037 to Christie to prepare preimpregnated strands of fibers wherein yarn or glass roving or the like is coated only with thermosetting resin and then used in conjunction with yarn, glass roving, etc., coated only with the curing agent. It is suggested therein that the coated filaments can be stored separately for long periods before use. When the two coated filaments are combined for use, however, the curing agent is not dispersed throughout the resin. As noted above, problems associated with slow diffusion of the curing agent into the resin and non-uniform polymerization can result. Moreover, in this technique heating is required, since curing proceeds at a reasonable rate only if carried out at an elevated temperature.
Other attempts have been made to provide systems for forming rigid polyester reaction products in which components of the resin system are storable or have good pot life, yet react quickly upon contact. Invariably, the curing agent is segregated from the resin which, as discussed above, leads to inferior curing characteristics resulting from the slow and non-uniform dispersion of the curing agent into the resin upon contact. Of this genre, in addition to U.S. Pat. No. 3,390,037, is U.S. Pat. No. 3,037,900, directed to providing a viscous air tight seal on the surface of the thermosetting resin shown as being spray applied in two components. Curing agent is included with the resin in one component, but is segregated from the resin in the second component. In U.S. Pat. No. 2,968,335, curing agent is included with the resin in one of two spray-applied components, but is not included in the second component. Accelerator is included in the second component, but not in the first. Another approach is seen in U.S. Pat. No. 3,914,200 directed to the use of two component system. A polyester resin is used with an excessive amount of inhibitors and a peroxide curing agent in the first of two resin baths. The second bath is a certain mixture of accelerators effective to accelerate the curing agent. In use, the resin is contacted with the accelerators to overcome the inhibitor. The added expense and processing complications of the inhibitor obviously render this approach disadvantageous in many applications.
Directed to a different subject matter, i.e., adhesive systems, is U.S. Pat. No. 3,725,501 wherein the curing agent for each of two distinct resins (e.g., polysilicone and unsaturated polyester) is segregated therefrom by incorporation into the other resin. In use, the two components would be mixed and, again, the curing agent would be effective only as and when it migrated or diffused into the opposite resin. Another approach is seen in U.S. Pat. No. 2,557,826 wherein the two components of an adhesive each contain phenol-aldehyde resin miscible with the other, and each carries the catalyst for the other. This, of course, fails to address or resolve the problems outlined above particular to the use of the peroxide curing agents in polyester type resins.
It is the object of the present invention, therefore, to provide a novel method of making a structural device, laminate, coating or the like employing thermosetting resin impregnated filaments, wherein the thermosetting resins employed are stable at room temperature and yet provide a structural preform which cures very rapidly without external heating. Furthermore, it is an object of this invlention to provide such method wherein suitable curing agent is uniformly distributed throughout the resin with which the filaments which form the structural preform are impregnated. In addition, it is an object to provide compositions of matter and methods of employing same for manufacture of structural preforms having large cross-sectional areas to which external heat can be applied to accelerate curing while substantially avoiding non-uniform cure and the disadvantages thereof. Thus, it is an object of the invention to provide a method of making a product having significantly improved physical properties.
It is a related object of this invention to provide a new composition of matter which comprises an intermediate product of such novel method. More specifically, it is an object of the invention to provide impregnated filaments suitable for making a structural preform, laminate or like product wherein the resin system comprises thermosetting resin, curing agent dispersed uniformly throughout the resin and accelerator.
In addition, it is an object to provide a novel method of making such new composition of matter.